Glymphatic system dysfunction in neurodegenerative diseases

PURPOSE OF REVIEW

Purpose of this review is to update the ongoing work in the field of glymphatic and neurodegenerative research and to highlight focus areas that are particularly promising.

RECENT FINDINGS

Multiple reports have over the past decade documented that glymphatic fluid transport is broadly suppressed in neurodegenerative diseases. Most studies have focused on Alzheimer's disease using a variety of preclinical disease models, whereas the clinical work is based on various neuroimaging approaches. It has consistently been reported that brain fluid transport is impaired in patients suffering from Alzheimer's disease compared with age-matched control subjects.

SUMMARY

An open question in the field is to define the mechanistic underpinning of why glymphatic function is suppressed. Other questions include the opportunities for using glymphatic imaging for diagnostic purposes and in treatment intended to prevent or slow Alzheimer disease progression.

A SPECT-based method for dynamic imaging of the glymphatic system in rats

The glymphatic system is a brain-wide waste drainage system that promotes cerebrospinal fluid circulation through the brain to remove waste metabolites. Currently, the most common methods for assessing glymphatic function are ex vivo fluorescence microscopy of brain slices, macroscopic cortical imaging, and MRI. While all these methods have been crucial for expanding our understanding of the glymphatic system, new techniques are required to overcome their specific drawbacks. Here, we evaluate SPECT/CT imaging as a tool to assess glymphatic function in different anesthesia-induced brain states using two radiolabeled tracers, [111In]-DTPA and [99mTc]-NanoScan. Using SPECT, we confirmed the existence of brain state-dependent differences in glymphatic flow and we show brain state-dependent differences of CSF flow kinetics and CSF egress to the lymph nodes. We compare SPECT and MRI for imaging glymphatic flow and find that the two imaging modalities show the same overall pattern of CSF flow, but that SPECT was specific across a greater range of tracer concentrations than MRI. Overall, we find that SPECT imaging is a promising tool for imaging the glymphatic system, and that qualities such as high sensitivity and the variety of available tracers make SPECT imaging a good alternative for glymphatic research.

Deletion of SERF2 in mice delays embryonic development and alters amyloid deposit structure in the brain

In age-related neurodegenerative diseases, like Alzheimer's and Parkinson's, disease-specific proteins become aggregation-prone and form amyloid-like deposits. Depletion of SERF proteins ameliorates this toxic process in worm and human cell models for diseases. Whether SERF modifies amyloid pathology in mammalian brain, however, has remained unknown. Here, we generated conditional Serf2 knockout mice and found that full-body deletion of Serf2 delayed embryonic development, causing premature birth and perinatal lethality. Brain-specific Serf2 knockout mice, on the other hand, were viable, and showed no major behavioral or cognitive abnormalities. In a mouse model for amyloid-β aggregation, brain depletion of Serf2 altered the binding of structure-specific amyloid dyes, previously used to distinguish amyloid polymorphisms in the human brain. These results suggest that Serf2 depletion changed the structure of amyloid deposits, which was further supported by scanning transmission electron microscopy, but further study will be required to confirm this observation. Altogether, our data reveal the pleiotropic functions of SERF2 in embryonic development and in the brain and support the existence of modifying factors of amyloid deposition in mammalian brain, which offer possibilities for polymorphism-based interventions.

Pretargeted Imaging beyond the Blood–Brain Barrier—Utopia or Feasible?

Pretargeting is a promising nuclear imaging technique that allows for the usage of antibodies (Abs) with enhanced imaging contrast and reduced patient radiation burden. It is based on bioorthogonal chemistry with the tetrazine ligation—a reaction between trans-cyclooctenes (TCOs) and tetrazines (Tzs)—currently being the most popular reaction due to its high selectivity and reactivity. As Abs can be designed to bind specifically to currently ‘undruggable’ targets such as protein isoforms or oligomers, which play a crucial role in neurodegenerative diseases, pretargeted imaging beyond the BBB is highly sought after, but has not been achieved yet. A challenge in this respect is that large molecules such as Abs show poor brain uptake. Uptake can be increased by receptor mediated transcytosis; however, it is largely unknown if the achieved brain concentrations are sufficient for pretargeted imaging. In this study, we investigated whether the required concentrations are feasible to reach. As a model Ab, we used the bispecific anti-amyloid beta (Aβ) anti-transferrin receptor (TfR) Ab 3D6scFv8D3 and conjugated it to a different amount of TCOs per Ab and tested different concentrations in vitro. With this model in hand, we estimated the minimum required TCO concentration to achieve a suitable contrast between the high and low binding regions. The estimation was carried out using pretargeted autoradiography on brain sections of an Alzheimer’s disease mouse model. Biodistribution studies in wild-type (WT) mice were used to correlate how different TCO/Ab ratios alter the brain uptake. Pretargeted autoradiography showed that increasing the number of TCOs as well as increasing the TCO-Ab concentration increased the imaging contrast. A minimum brain concentration of TCOs for pretargeting purposes was determined to be 10.7 pmol/g in vitro. Biodistribution studies in WT mice showed a brain uptake of 1.1% ID/g using TCO-3D6scFv8D3 with 6.8 TCO/Ab. According to our estimations using the optimal parameters, pretargeted imaging beyond the BBB is not a utopia. Necessary brain TCO concentrations can be reached and are in the same order of magnitude as required to achieve sufficient contrast. This work gives a first estimate that pretargeted imaging is indeed possible with antibodies. This could allow the imaging of currently ‘undruggable’ targets and therefore be crucial to monitor (e.g., therapies for intractable neurodegenerative diseases).

An in vivo Pig Model for Testing Novel Positron Emission Tomography Radioligands Targeting Cerebral Protein Aggregates

Positron emission tomography (PET) has become an essential clinical tool for diagnosing neurodegenerative diseases with abnormal accumulation of proteins like amyloid-β or tau. Despite many attempts, it has not been possible to develop an appropriate radioligand for imaging aggregated α-synuclein in the brain for diagnosing, e.g., Parkinson’s Disease. Access to a large animal model with α-synuclein pathology would critically enable a more translationally appropriate evaluation of novel radioligands. We here establish a pig model with cerebral injections of α-synuclein preformed fibrils or brain homogenate from postmortem human brain tissue from individuals with Alzheimer’s disease (AD) or dementia with Lewy body (DLB) into the pig’s brain, using minimally invasive surgery and validated against saline injections. In the absence of a suitable α-synuclein radioligand, we validated the model with the unselective amyloid-β tracer [¹¹C]PIB, which has a high affinity for β-sheet structures in aggregates. Gadolinium-enhanced MRI confirmed that the blood-brain barrier was intact. A few hours post-injection, pigs were PET scanned with [¹¹C]PIB. Quantification was done with Logan invasive graphical analysis and simplified reference tissue model 2 using the occipital cortex as a reference region. After the scan, we retrieved the brains to confirm successful injection using autoradiography and immunohistochemistry. We found four times higher [¹¹C]PIB uptake in AD-homogenate-injected regions and two times higher uptake in regions injected with α-synuclein-preformed-fibrils compared to saline. The [¹¹C]PIB uptake was the same in non-injected (occipital cortex, cerebellum) and injected (DLB-homogenate, saline) regions. With its large brain and ability to undergo repeated PET scans as well as neurosurgical procedures, the pig provides a robust, cost-effective, and good translational model for assessment of novel radioligands including, but not limited to, proteinopathies.

Pretargeted Imaging Beyond the Blood-Brain Barrier

Pretargeting is a powerful nuclear imaging strategy to achieve enhanced imaging contrast for nanomedicines and reduce the radiation burden to healthy tissue. Pretargeting is based on bioorthogonal chemistry. The most...

Early Aβ reduction prevents progression of cerebral amyloid angiopathy

OBJECTIVE

Clinical trials targeting β-amyloid peptides (Aβ) for Alzheimer disease (AD) failed for arguable reasons that include selecting the wrong stages of AD pathophysiology or Aβ being the wrong target. Targeting Aβ to prevent cerebral amyloid angiopathy (CAA) has not been rigorously followed, although the causal role of Aβ for CAA and related hemorrhages is undisputed. CAA occurs with normal aging and to various degrees in AD, where its impact and treatment is confounded by the presence of parenchymal Aβ deposition.

METHODS

APPDutch mice develop CAA in the absence of parenchymal amyloid, mimicking hereditary cerebral hemorrhage with amyloidosis Dutch type (HCHWA-D). Mice were treated with a β-site amyloid precursor protein cleaving enzyme 1 (BACE1) inhibitor. We used 3-dimensional ultramicroscopy and immunoassays for visualizing CAA and assessing Aβ in cerebrospinal fluid (CSF) and brain.

RESULTS

CAA onset in mice was at 22 to 24 months, first in frontal leptomeningeal and superficial cortical vessels followed by vessels penetrating the cortical layers. CSF Aβ increased with aging followed by a decrease of both Aβ40 and Aβ42 upon CAA onset, supporting the idea that combined reduction of CSF Aβ40 and Aβ42 is a specific biomarker for vascular amyloid. BACE1 inhibitor treatment starting at CAA onset and continuing for 4 months revealed a 90% Aβ reduction in CSF and largely prevented CAA progression and associated pathologies.

INTERPRETATION

This is the first study showing that Aβ reduction at early disease time points largely prevents CAA in the absence of parenchymal amyloid. Our observation provides a preclinical basis for Aβ-reducing treatments in patients at risk of CAA and in presymptomatic HCHWA-D. ANN NEUROL 2019;86:561-571.